Deterministic Integration of hBN Single-Photon Emitters on SiN Waveguides via Femtosecond Laser Processing

TL;DR

The paper addresses the challenge of integrating stable two-dimensional material SPEs onto established photonic chips. It introduces a deterministic post-fabrication approach: dry-transfer of hBN flakes onto SiN waveguides followed by localized femtosecond-laser irradiation to form optically active defects, then annealing to activate them. Among the created defects, at least one (S3) displays narrow-line emission with polarization dependence and clear antibunching, with successful on-chip excitation via the SiN waveguide. The approach provides a scalable route to hybrid 2D material photonics compatible with mature silicon nitride platforms, enabling on-chip quantum photonics with potential for scalable networks.

Abstract

We demonstrate a post-fabrication method that deterministically integrates hexagonal boron nitride (hBN) single-photon emitters (SPEs) onto silicon nitride (SiN) waveguides. Mechanically exfoliated hBN flakes are dry-transferred onto pre-fabricated SiN waveguides, and localized femtosecond laser irradiation is employed to induce defects with sub-microscale spatial precision. Confocal photoluminescence mapping reveals multiple laser-written bright defects, among which one emitter exhibits narrow spectral linewidth and polarization dependence characteristic of a dipole emitter. The emitter exhibits high brightness and temporal stability, and second-order photon correlation measurements confirm its single-photon nature. Furthermore, we successfully achieve on-chip excitation via the SiN waveguide, demonstrating the compatibility of our approach with mature photonic platform technologies. This deterministic integration technique offers a scalable pathway for incorporating quantum emitters into photonic circuits, paving the way for the development of quantum information processing and communication systems with two-dimensional material hybrid photonic devices.

Figures (4)

• Figure 1: (a) Schematic illustration of the deterministic post-fabrication method based on femtosecond laser processing for integrating hBN SPEs on SiN waveguides. (b) Optical microscope image of the device showing an hBN flake transferred onto a SiN waveguide equipped with grating couplers at both ends. The green-colored area indicates the hBN flake. The scale bar corresponds to 5 $\mathrm{\mu}$m. (c) Confocal PL map of the region indicated by the dashed box in panel (b) is shown. Four laser-written spots, referred to as S1–S4, are clearly visible. The excitation polarization is set to 90$^{\circ}$ (vertical direction), and the laser power is 2.5 mW. The scale bar is 2 $\mathrm{\mu}$m.
• Figure 2: (a–d) PL spectra obtained from S1–S4 defects as indicated in Fig. \ref{['Fig1']}(c). In the green curves, the sharp peaks around 500 nm correspond to Raman signals from silicon (500 nm), SiN (512 nm), and hBN (524 nm), while the broad peaks centered at 500–600 nm originate from PL of hBN. The gray curves are background emission from a bare SiN waveguide. For each defect, the excitation polarization angle is adjusted to the value yielding maximum PL intensity, as shown in the respective insets. In the device image of Fig. \ref{['Fig1']}(b), the angle of 0$^{\circ}$ corresponds to horizontal polarization. The polarization curve for the S3 defect is fitted using the equation $\cos^2(\theta)$, and the fit yielded an angle $\theta = 16^\circ$. The excitation power for all measurements is 2.5 mW. (e–h) Second-order correlation functions $g^{(2)}(\tau)$ mesured for S1–S4 defects. Experimental data are shown in gray, and the fitted curves are in green. The fitting is performed using the equation $g^{(2)}(\tau) = 1 - a e^{-|\tau|/\tau_{1}}$. The excitation power is 2.5 mW for the S1, S2, and S4 defects, and 1.0 mW for the S3 defect. The dependence of $g^{(2)}(\tau)$ on excitation power for the S3 defect is presented in Fig. S3 of the Supporting Information. Antibunching is observed for the S3 and S4 defects, with $g^{(2)}(0) = 0.28 \pm 0.10$ and $0.66 \pm 0.07$, respectively.
• Figure 3: (a) PL count rate from the S3 defect as a function of excitation power. Data points are shown as circles, and the green line represents a fit using the standard saturation model. (b) Time-trace PL intensity of S3 at 1 mW excitation power, demonstrating stable emission. The time bin width is set to 200 ms. (c) AFM image of the S3 defect. The green scale bar is 1.5 $\mathrm{\mu}$m. (d) Height profile along the green line in panel (c). Measured data points are plotted as crosses, and the green curve indicates a box-averaged profile using a 5-point moving window.
• Figure 4: (a) PL intensity map measured by scanning the excitation position using a 4f beam-steering system, while keeping the collection point fixed at the S3 defect. The scale bar is 2 $\mathrm{\mu}$m. (b) Second-order correlation function $g^{(2)}(\tau)$ of the S3 defect under waveguide excitation. In this configuration, excitation light is coupled into the right-side grating coupler, and emission is collected at the defect spot. Experimental data and the corresponding fit are shown in gray and green, respectively. The data is fitted using the equation $g^{(2)}(\tau) = 1 - a e^{-|\tau|/\tau_{1}}$, yielding $g^{(2)}(0) = 0.26 \pm 0.08$. The excitation power is 2.5 mW. The lower-left inset shows a schematic diagram of the excitation and collection configuration. The lower-right inset displays the polarization dependence of the PL intensity under waveguide excitation. Measured data are plotted as circles, and the green line represents a fit using the equation $\cos^2(\theta)$, yielding an angle $\theta = 64^\circ$.